Particle accelerators are used to increase the energy of electrically-charged atomic particles, e.g., electrons, protons, or charged atomic nuclei, so that they can be studied by nuclear and particle physicists. High energy electrically-charged atomic particles are accelerated to collide with target atoms, and the resulting products are observed with a detector. At very high energies the charged particles can break up the nuclei of the target atoms and interact with other particles. Transformations are produced that tip off the nature and behavior of fundamental units of matter. Particle accelerators are also important tools in the effort to develop nuclear fusion devices, as well as for medical applications such as cancer therapy.
One type of particle accelerator is disclosed in U.S. Pat. No. 5,757,146 to Carder, incorporated by reference herein, for providing a method to generate a fast electrical pulse for the acceleration of charged particles. In Carder, a dielectric wall accelerator (DWA) system is shown consisting of a series of stacked circular modules which generate a high voltage when switched. Each of these modules is called an asymmetric Blumlein, which is described in U.S. Pat. No. 2,465,840 incorporated by reference herein. As can be best seen in FIGS. 4A-4B of the Carder patent, the Blumlein is composed of two different dielectric layers. On each surface and between the dielectric layers are conductors which form two parallel plate radial transmission lines. One side of the structure is referred to as the slow line, the other is the fast line. The center electrode between the fast and slow line is initially charged to a high potential. Because the two lines have opposite polarities there is no net voltage across the inner diameter (ID) of the Blumlein. Upon applying a short circuit across the outside of the structure by a surface flashover or similar switch, two reverse polarity waves are initiated which propagate radially inward towards the ID of the Blumlein. The wave in the fast line reaches the ID of the structure prior to the arrival of the wave in the slow line. When the fast wave arrives at the ID of the structure, the polarity there is reversed in that line only, resulting in a net voltage across the ID of the asymmetric Blumlein. This high voltage will persist until the wave in the slow line finally reaches the ID. In the case of an accelerator, a charged particle beam can be injected and accelerated during this time. In this manner, the DWA accelerator in the Carder patent provides an axial accelerating field that continues over the entire structure in order to achieve high acceleration gradients.
The existing dielectric wall accelerators, such as the Carder DWA, however, have certain inherent problems which can affect beam quality and performance. In particular, several problems exist in the disc-shaped geometry of the Carder DWA which make the overall device less than optimum for the intended use of accelerating charged particles. The flat planar conductor with a central hole forces the propagating wavefront to radially converge to that central hole. In such a geometry, the wavefront sees a varying impedance which can distort the output pulse, and prevent a defined time dependent energy gain from being imparted to a charged particle beam traversing the electric field. Instead, a charged particle beam traversing the electric field created by such a structure will receive a time varying energy gain, which can prevent an accelerator system from properly transporting such beam, and making such beams of limited use.
Additionally, the impedance of such a structure may be far lower than required. For instance, it is often highly desirable to generate a beam on the order of milliamps or less while maintaining the required acceleration gradients. The disc-shaped Blumlein structure of Carder can cause excessive levels of electrical energy to be stored in the system. Beyond the obvious electrical inefficiencies, any energy which is not delivered to the beam when the system is initiated can remain in the structure. Such excess energy can have a detrimental effect on the performance and reliability of the overall device, which can lead to premature failure of the system.
And inherent in a flat planar conductor with a central hole (e.g. disc-shaped) is the greatly extended circumference of the exterior of that electrode. As a result, the number of parallel switches to initiate the structure is determined by that circumference. For example, in a 6″ diameter device used for producing less than a 10 ns pulse typically requires, at a minimum, 10 switch sites per disc-shaped asymmetric Blumlein layer. This problem is further compounded when long acceleration pulses are required since the output pulse length of this disc-shaped Blumlein structure is directly related to the radial extent from the central hole. Thus, as long pulse widths are required, a corresponding increase in switch sites is also required. As the preferred embodiment of initiating the switch is the use of a laser or other similar device, a highly complex distribution system is required. Moreover, a long pulse structure requires large dielectric sheets for which fabrication is difficult. This can also increase the weight of such a structure. For instance, in the present configuration, a device delivering 50 ns pulse can weigh as much as several tons per meter. While some of the long pulse disadvantages can be alleviated by the use of spiral grooves in all three of the conductors in the asymmetric Blumlein, this can result in a destructive interference layer-to-layer coupling which can inhibit the operation. That is, a significantly reduced pulse amplitude (and therefore energy) per stage can appear on the output of the structure.
Additionally, various types of accelerators have been developed for particular use in medical therapy applications, such as cancer therapy using proton beams. For example, U.S. Pat. No. 4,879,287 to Cole et al discloses a multi-station proton beam therapy system used for the Loma Linda University Proton Accelerator Facility in Loma Linda, California. In this system, particle source generation is performed at one location of the facility, acceleration is performed at another location of the facility, while patients are located at still other locations of the facility. Due to the remoteness of the source, acceleration, and target from each other particle transport is accomplished using a complex gantry system with large, bulky bending magnets. And other representative systems known for medical therapy are disclosed in U.S. Pat. No. 6,407,505 to Bertsche and U.S. Pat. No. 4,507,616 to Blosser et al. In Berstche, a standing wave RF linac is shown and in Blosser a superconducting cyclotron rotatably mounted on a support structure is shown.
Furthermore, ion sources are known which create a plasma discharge from a low pressure gas within a volume. From this volume, ions are extracted and collimated for acceleration into an accelerator. These systems are generally limited to extracted current densities of below 0.25 A/cm2. This low current density is partially due to the intensity of the plasma discharge at the extraction interface. One example of an ion source known in the art is disclosed in U.S. Pat. No. 6,985,553 to Leung et al having an extraction system configured to produce ultra-short ion pulses. Another example is shown in U.S. Pat. No. 6,759,807 to Wahlin disclosing a multi-grid ion beam source having an extraction grid, an acceleration grid, a focus grid, and a shield grid to produce a highly collimated ion beam.